Environ. Sci. Technol. 1987, 27, 209-210
NOTES TITRATOR: An Interactive Program for Aquatic Equilibrium Calculations Stephen E. Cabaniss
Department of Environmental Sciences and Engineering, University of North Carolina at Chapel Hill, Chapel HIII, North Carolina 27514 The interactive microcomputer program TITRATOR calculates equilibrium speciation for aqueous solutions of up to 130 dissolved, solid, and gaseous species. The program is very general and easy to use and allows convenient simulation of laboratory and environmental solutions. Two sample calculations show the speciation of iron in a model deaerated groundwater as a function of pH and the sensitivity of A1 speciation in a simulated acid lake water to the approximation of Kformation for AlOHF’.
Introduction The complexity of aqueous chemistry can obscure fundamental relationships, making it difficult to see the forest for the trees. Equilibrium calculations that are rapid and intuitive for simple solutions are unwieldy and laborious when a dozen or more species concentrations must be determined. Computer calculation of speciation frees researchers to study chemical relationships. A number of aquatic chemical equilibrium programs have been applied to environmental systems (1,2).This note describes an interactive microcomputer program for equilibrium calculations and its application to an environmental problem. The general equilibrium problem is solved by finding the set of species concentrations that minimizes the Gibbs free energy and satisfies mass balance for a given set of chemical components. Most programs provide a thermodynamic data base so the user need not select and enter stability constants. Programs without a data base may be less convenient, but encourage critical evaluation of the data base (3). In 1976, the MINEQL user’s manual ( 4 ) stated ‘‘MINEQL ...requires a full size computer (128 K) for execution”. In 1986, the standard configuration of an IBM-PC microcomputer includes 256 K of memory. MICROQL (3) and MINEQL ( 4 ) can take advantage of the ready availability and low cost of microcomputers but do not use the interactive mode or graphical displays that have made some programs so popular. Program Description TITRATOR is an interactive program for exploring the relationships among variables in chemical equilibria. The menu-driven user interface and help screens simplify learning and use. Model solutions are created or modified via an editor that prompts for all necessary information. TITRATOR calculates solution speciation for a single set of parameters or as a function of one variable (e.g., total acid, pH, pE, log KfOmtion, ionic strength, etc.). Titration results are displayed in tables and graphs and may be transferred to other programs via ASCII files. A principal advantage of TITRATOR relative to many batch programs is that it ‘‘titrates” and plots the data 0013-936X/87/0921-0209$01.50/0
without any user programming, This flexibility is most useful for initial examination of equilibria-calculating activity ranges, selecting important variables, sensitivity analysis, and “what if” problems. TITRATOR is much slower than mainframe programs, which may be a drawback for large equilibrium systems; however, longer solution time on a microcomputer may be partly or entirely offset by the longer access queues and response times on crowded mainframe systems. TITRATOR solves equilibrium problems by mass balance error minimization with a Newton-Raphson algorithm (4). The user specifies dissolved, solid, or gaseous components by either fixed activity (pH, pC0,) or fixed total amount (total strong acid, total carbonate species). No comprehensive thermodynamic data set is supplied. The user may store data sets of individual solutions for later use or modification. The program calculates activity coefficients from the Guntelberg equation at low (
